The invention relates to a method for obtaining a composition comprising an aromatic polyamide containing para-phenylene terephthalamide and 2-(p-phenylene)-benzimidazole terephthalamide units by copolymerizing para-phenylenediamine (PPD), 5(6)-amino-2-(p-aminophenyl)benzimidazole (DAPBI); and terephthaloyl dichloride (TDC) in a mixture of N-methyl pyrrolidone (NMP) and calcium chloride, and to said composition. The invention further relates to a method for making purified aromatic polymer from said composition.
Methods for making aramid polymers are known in the art. For instance, in U.S. Pat. No. 4,172,938, an aromatic polyamide was described to be made by polymerizing a mixture of diamines and an aromatic dicarboxylic acid dihalide in a mixture of N-methyl pyrrolidone and calcium chloride. In example 34 of this reference, the polymerization reaction is performed with a mixture of para-phenylene diamine (PPD) and 5-amino-2-(p-aminophenyl)-benzimidazole (DAPBI), and terephthaloyl dichloride (TDC) in N-methyl pyrrolidone (NMP) containing 2 wt. % of calcium chloride (CaCl2). The product was obtained as a powdery clay-like material for which filtration was problematic. It was disclosed that products according to this reference in more general terms are obtained as slurry, paste, powder, or agar.
Other processes for making spin dopes of DAPBI-containing polymers are known from U.S. Pat. No. 5,646,234 and U.S. Pat. No. 4,018,735. U.S. Pat. No. 5,646,234 discloses a process for making a spin dope wherein the use of alkali metal halides, among which also calcium chloride, as additive is disclosed. However, very particular preference is given in using no additives, and in conformity herewith the specific examples do not use such additives. Moreover, if calcium chloride is applied according to this reference the amount thereof can be substantially higher than allowable for preventing formation of paste and the like. None of the examples of U.S. Pat. No. 4,018,735 disclose the use of calcium chloride, nor is such specific additive suggested in combination with N-methyl pyrrolidone, and for that reason the polymers of this reference will be obtained in the form of a paste, powder, and the like.